The present invention relates to an apparatus for treatment of radioactive waste liquid generated in a nuclear power plant or the like and, more particularly, to a radioactive waste liquid treatment apparatus of a non-boiling type, i.e., of a membrane-type vaporization/concentration system.
Radioactive waste liquid inevitably produced in a nuclear power plant is an aggregate material consisting of waste liquid produced in regeneration of a demineralizer, floor drain and the like. Such waste liquid includes, as a main component, a large amount of sodium sulfate Na.sub.2 SO.sub.4 in the case of a BWR (boiling water reactor) plant. In the case of a PWR (pressurized water reactor) plant, the waste liquid includes a large amount of sodium borate Na.sub.3 B.sub.4 O.sub.7, having a high electric conductivity of 5 ms/cm.
A main component of waste liquid produced in a spent fuel reprocessing plant is sodium nitrate NaNO.sub.3, its electric conductivity being similarly high.
Since the waste liquid of such a high electric conductivity cannot be directly processed in a demineralizer, the waste liquid is concentrated in a vaporization/concentration apparatus to the extent that Na.sub.2 SO.sub.4 will be about 25 wt %. Likewise, Na.sub.3 B.sub.4 O.sub.7 will be concentrated to about 12 wt %. On the other hand, vaporized water is condensed and turned into water, however, its electric conductivity is still high. Consequently, the condensed water is further processed in a demineralizer so as to be recycled for makeup water.
In this case, Na.sub.2 SO.sub.4 as a scale component and chlorine ions as a corrosive component are highly concentrated in the vaporization/concentration apparatus, and therefore, there are taken strong measures against corrosion of structural material of the apparatus. For instance, titanium is used for the structural material. Further concerning corrosion resistance, processing is done under reduced pressure to boil water at a temperature as low as possible, e.g., 65.degree. C. It is also necessary to take such steps as to mix an anticorrosive called "inhibitor" with processing liquid as a pre-processing.
Besides, boiling vaporization in the vaporization/concentration system induces mist to disperse to a large extent and increases a load of the demineralizer which is provided for maintaining purity of the condensed water.
For the reasons, there has been suggested a membrane-type vaporization/concentration system in which a hydrophobic porous membrane is used for a vaporization surface to perform non-boiling vaporization/concentration.
In the membrane-type vaporization/concentration system, as disclosed in Japanese Patent Unexamined Publications Nos. 61-286798, 61-164195 and the like, there is employed a porous membrane of hydrophobic polymer through which water vapor is transferred to a space where it is condensed, whereas the membrane prevents ions and the like in waste liquid from passing therethrough, to thereby concentrate the waste liquid.
However, radioactive waste liquid usually contains detergent drainage or the like including interfacial active agents, and such interfacial active agents degrade the function of the above-mentioned porous membrane. That is, as indicated by a continuous curve line of a graph in FIG. 6, when concentration of interfacial active agents contained in waste liquid is 3.5 mg/l or more, the electric conductivity of condensed water becomes drastically increased. The reason seems to be that hydrophobic property of the membrane is damaged because surface tension of the waste liquid is lowered and because the interfacial active agents are adhered on the membrane, so that the waste liquid in the original state may leak through the membrane.
In addition, radioactive waste liquid usually contains machine oil or the like including oil and solid matter, which also deteriorates the function of the porous membrane.